The present invention relates to a process for preparing a lithium nickel cobalt complex oxide having a high purity, a high crystallinity, a high battery capacity and stable structure so that the degree of decrease in the capacity is little even with increasing the number of charging and discharging cycles, and to a positive electrode active material for a secondary battery which contains said lithium nickel cobalt complex oxide as an effective ingredient.
As electronic appliances have been rendered small and portable in recent years, there has been increased a demand for a lithium ion secondary battery having a light weight and a high energy density in place of nickel/cadmium battery and nickel hydrogen battery.
As active materials of a positive electrode for this lithium ion secondary battery, there are known LiNiO2 and LiCoO2 which are layered compounds capable of intercalating and deintercalating lithium ions. Of them, LiNiO2 is preferred due to its higher electrical capacity than LiCoO2.
However, LiNiO2 has not yet been put to practical use because it has problems in the charging and discharging cycle characteristics, the storage stability and the stability at a high temperature. Only LiCoO2 has been practically used as the positive electrode active material.
Although various attempts have been made to improve the above faults of LiNiO2 for its utilization as the positive electrode active material for a secondary battery, there has not yet been realized one wherein all of the above faults have been solved.
That is, in case of LiNiO2 it is known that when many lithium ions are liberated therefrom (during charge), the structure becomes unstable owing to the two dimensional structure and therefore the cycle property, storage stability and high temperature stability of the lithium ion secondary battery are poor [for example, see J. Electrochem. Soc., 140 [7] p. 1862-1870 (1993), Solid State Ionics, 69 p. 265-70 (1994)]. Although many attempts have been made to stabilize the structure by replacing a portion of Ni with other components (Co, Mn, Fe, Ti, V etc.) for the purpose of securing the structure stability with elimination of the above faults, it was difficult to obtain highly purified and completely doped crystals as a solid solution on an industrial scale because there have been practically applied dry blending and heating processes.
Also, an attempt has been made to control to certain specific levels of the physical properties such as the shape and size of LiNiO2 particles and its doped product with other components as solid solution. However, satisfactory results could not be achieved. For example, Japanese Patent Laid-open No. 151998/1993 proposes an improvement wherein the particle size distribution is specified to such extent that 10% cumulative size is 3xcx9c15 xcexcm, 50% cumulative size 8xcx9c35 xcexcm and 90% cumulative size 30xcx9c80 xcexcm. However, it is very difficult to adjust the particle size distribution to such an extent by grinding the positive electrode active material.
Usually, LiNiO2 has been prepared by mixing lithium components (LiOH, Li2CO3, LiNiO3 etc.) with nickel components (hydroxide, carbonate etc.) in a dry state and thereafter subjecting the mixture to the reaction, and hence the heating at an elevated temperature for a long time was required. Consequently, the crystal growth proceeds but some of lithium is evaporated off and NiO as a by-product is formed, thereby lowering the purity.
Therefore, it was difficult to prepare highly purified product by the dry process in cases where the primary particle size is small. On the other hand, in cases where the primary particle size is large, a considerable lattice defect in the structure is caused resulting in a lowering of the purity. It was impossible to adjust crystal size as desired while keeping crystallinity and purity at high levels by the dry process.
An object of the present invention is to provide a process for preparing a lithium nickel cobalt complex oxide which has improved properties with respect to the above faults of the hitherto known LiNiO2 and its related complex oxide, namely which has a high purity, a high crystallinity, a high battery capacity and stable structure so that the degree of decrease in the capacity is little even by increasing the number of charging and discharging cycles.
Another object of the present invention is to provide a process for preparing said lithium nickel cobalt complex oxide via wet process which is different from the hitherto known dry process whereby the size of the formed spherical and secondary and primary particle may be set to a desired size.
A further object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery containing as an effective component said lithium nickel cobalt complex oxide.
As a result of having studied ardently to achieve the above objects, the present inventors have found that a complex oxide which may be represented by the following general formula (I) and which may be prepared at the first time by a wet process described later coincides with the above objects;
LiyN1xe2x88x92xCox1Mx2O2xe2x80x83xe2x80x83(I)
(wherein M represents at least one element selected from the group consisting of Al, Fe, Mn and B. y represents 0.9xe2x89xa6yxe2x89xa61.3, x1+x2=x, x represents 0 less than xxe2x89xa60.5, x1 represents 0 less than x1 less than 0.5; when M is at least one element among Al, Fe and Mn, x2 represents 0 less than x2xe2x89xa60.3; when M is B, x2 represents 0 less than x2 less than 0.1 and when M is a combination of B and at least one element among Al, Fe and Mn, x2 represents 0 less than x2 less than 0.3 wherein the proportion occupied by B is in the range of being larger than 0 but being smaller than 0.1).
The complex oxide which may be obtained by the process of the present invention has the following features.
The first feature lies in the composition represented by the above general formula (I).
Holding as high a battery capacity as LiNiO2, the disclosed composition has improved cycle properties (i.e. lowered deterioration of discharge capacity when increasing the number of cycles), high temperature stability, and uses less expensive cobalt.
The second feature of the complex oxide lies in having a high crystallinity and a high purity as identified by its X-ray diffraction pattern. That is, it is highly purified complex oxide to such extent that an X-ray diffraction pattern shows that a ratio in the peak intensity of the face (003) to the face (104) i.e., (003)/(104) is 1.2 or higher and a ratio in the peak intensity of the face (006) to the face (101) i.e., (006)/(101) is 0.13 or lower, said face being defined by Miller indices hkl, the proportion of (Ni3xe2x88x92+Co3+) to the total (Ni+Co) being 99% by weight or higher, a BET specific surface area being 0.1xcx9c2 m2/g, an average secondary particle size D being in the range of 5xcx9c100 xcexcm with 10% of the particle size distribution being 0.5D or higher and 90% 2D or lower, the surface of the spherical secondary particle being uneven as observed with a scanning electron microscope (SEM) and the primary particle constituting the spherical secondary particle being in the range of 0.2xcx9c30 xcexcm in terms of long diameter with the average diameter of 0.3xcx9c30 xcexcm as observed with a SEM.
In case of LiNiO2 and its related complex oxide, when a part of the Ni is intended to be doped with other component(s) as the solid solution it is difficult to dope them homogeneously by the hitherto known dry process because the homogeneity is lowered in proportion to the amount added of other component(s) whereby not only is the battery capacity lowered but also the improvement to be achieved in the cycle property, the heat resistance and the electrolytic solution resistance are insufficient.
The lithium nickel cobalt complex oxide which may be obtained by the process of the present invention can be kept in high purity, in spite of being one doped with at least one element selected from the group consisting of Al, Fe, Mn and B. As shown in Examples described later, the interlayer distance may be efficiently shortened especially by using Co together with Al and/or B whereby the structural instability of Ni by reversible deintercalation of lithium ions can be avoided. The greatest feature in the process of the present invention is that Co and at least one element selected from the group consisting of Al, Fe, Mn and B may be doped as solid solution in a small amount and uniformly into the lithium nickel complex oxide. Such lithium nickel cobalt complex oxide can be obtained as the composition having a high purity and a high crystallinity by the wet process as described later.
The third feature of the complex oxide of the present invention is that there can be obtained uniform and primary particle and that shape and size of the secondary particle may be adjusted to that desired.
When attention is paid to the size of the primary particle, in general the size of the primary particle is important for a layered compound represented by LiMO2 in the light of the reversible deintercalation of the lithium ion. The finer the primary particle, the better ionic conductivity in the inside of the solid and the lithium ion is more reversibly deintercalatable with the outside.
On the other hand, in considering the complex oxide from an aspect of the crystallization degree when the crystallization degree is small the crystal growth does not proceed sufficiently and the purity becomes low inevitably. Also, in the case that the primary particle is small the storage stability is poor owing to moisture absorpbility and so good battery characteristics can not be achieved stably. Moreover, it is desirable that the primary particle is large taking the high temperature resistance and the reactivity with the electrolytic solution into consideration. As a result of having studied ardently, the present inventors have succeeded in the preparation of the complex oxide having uniform primary particles of such that a long diameter of the primary particles is in the range of 0.2xcx9c30 xcexcm, preferably 1xcx9c20 xcexcm by combined wet process-spray (or freeze) drying process-press molding and heating processes as described later.
A complex oxide wherein both the primary and secondary particles are uniform may be prepared by employing especially spray drying-heating processes. A long diameter of the primary particles is in the range of 0.2xcx9c30 xcexcm, preferably 1xcx9c20 xcexcm and its average size is in the range of 0.3xcx9c30 xcexcm when observed with an SEM. An average size D of the spherical secondary particle formed by spray drying-heating processes is in the range of 5xcx9c300 xcexcm, preferably 5xcx9c100 xcexcm, more preferably 5xcx9c20 xcexcm and the particle is uniform to such extent that 10% of the particle size distribution is 0.5D or higher and 90% 2D or lower, and the surface of the spherical secondary particle is uneven as can be seen under observation of an SEM.
Also, the particle ratio (a ratio of the long diameter to the short diameter) of the spherical secondary particles when observed with an SEM lies in the range of a maximum of 1.5 or less and an average of 1.2 or less with 90% or more of them being distributed in 1.3 or less, indicating that they are uniform particles even when there was included some particles having slightly larger particle ratio than defined above in the complex oxide prepared by pulverization after the heating.
It is understood from such physical properties that not only the spherical product, preferably one which may be obtained by the spray drying-heating processes, is suitable for the closest packing density but also it has advantages when used as a battery that the contact surface with each an electrolyte and a conductive agent becomes large so it is easy to reversibly deintercalate Li ions with the outside.
The size of the spherical secondary particles can be set to the range of from 5 xcexcm to 100 xcexcm as desired. However, an average size of about 5xcx9c30 xcexcm is desirable for use as the battery material from the viewpoint of processibility. Also, the BET specific surface area lies in the range of 0.1xcx9c2 m2/g. When it was used as the battery material, since there is no increase in the viscosity of an electrolyte, it does not cause a lowering in conductivity.
Also, for the purpose of setting the average long diameter of the primary particle to the range of about 1 xcexcmxcx9c30 xcexcm it may be more simply and conveniently achieved by subjecting the spray (or freeze) dried product as abovementioned to press molding. In case that the primary particle is large, it has physical properties that the purity and the crystallinity degree are high and that the high temperature stability is excellent, and therefore it may be preferably used as the positive electrode active material for a secondary battery which would be used under a severe condition. The bulk density becomes large due to press molding being applied. That the bulk density is high is a plus for the elevation of the battery capacity.
The following illustrates a process for preparing the complex oxide represented by the above general formula (I) in accordance with the present invention.
In preparing the complex oxide represented by the above general formula (I), the following processes are applied according to the three kinds of classifications: {circle around (1)} M is at least one element of Al, Fe and Mn, {circle around (2)} M is B and {circle around (3 )} M is the combination of B and at least one element of Al, Fe and Mn.
That is, {circle around (1)} in a process for preparing a complex oxide represented by the general formula (I)
LiyNi1xe2x88x92xCox1lMx2O2xe2x80x83xe2x80x83(I)
(wherein M is at least one element selected from the group consisting of Al, Fe and Mn), said complex oxide may be prepared by adding an amount of a lithium compound corresponding to the number of atomic moles of Li indicated by y to a basic metal salt represented by the general formula (II)
xe2x80x83Ni1xe2x88x92xCox1Mx2(OH)2(1xe2x88x92x+x1)+3xc3x972xe2x88x92nz(Anxe2x88x92)z.mH2Oxe2x80x83xe2x80x83(II)
[wherein M represents at least one element selected from the group consisting of Al, Fe and Mn, x represents 0 less than xxe2x89xa60.5, x1 is 0 less than x1 less than 0.5, x2 represents 0 less than x2xe2x89xa60.3, x1+x2=x, Anxe2x88x92 represents an anion having a valence of n (n=1xcx9c3) and z and m are positive numbers respectively satisfying the ranges of 0.03xe2x89xa6zxe2x89xa60.3, 0xe2x89xa6m less than 2] in an aqueous medium to form a slurry, spray or freeze drying the formed slurry and heating the spray or freeze dried product at a temperature of about 600xc2x0 C.xcx9c900xc2x0 C. for 4 hours or more in an oxidative atmosphere.
{circle around (2)} In a process for preparing a complex oxide represented by the general formula (I)
LiyNi1xe2x88x92xCox1MxO2xe2x80x83xe2x80x83(I)
(wherein M represents B), said complex oxide may be prepared by adding a boron compound containing x2 mol % of boron [x2 represents 0 less than x2 less than 0.1, the relationship among x, x1 and x2 is expressed by x2=xxe2x88x92x1] to a basic metal salt represented by the general formula (III)
Ni1xe2x88x92xCox1(OH)2(1xe2x88x92x+x1)xe2x88x92nz(Anxe2x88x92)z.mH2Oxe2x80x83xe2x80x83(III)
[wherein x represents 0 less than xxe2x89xa60.5, x1 represents 0 less than x1 less than 0.5, Anxe2x88x92 represents an anion having a valence of n (n=1xcx9c3) and z and m represent positive numbers respectively satisfying the ranges of 0.03xe2x89xa6zxe2x89xa60.3, 0xe2x89xa6m less than 2] in an aqueous medium, subsequently adding thereto an amount of a lithium compound corresponding to the number of atomic moles of Li indicated by y to form a slurry, spray or freeze drying the formed slurry and heating the spray or freeze dried product at a temperature of about 600xc2x0 C.xcx9c900xc2x0 C. for 4 hours or more in an oxidative atmosphere.
{circle around (3)} In a process for preparing a complex oxide represented by the general formula (I)
LiyNi1xe2x88x92xCox1Mx2O2xe2x80x83xe2x80x83(I)
(wherein M represents the combination of B and at least one element selected from the group consisting of Al, Fe and Mn), said complex oxide may be prepared by adding a boron compound containing x4 mol % of boron [x4 represents 0 less than x4 less than 0.1, the relationship among x4, x3 and x2 is expressed by x4+x3=x2] and an amount of a lithium compound corresponding to the number of atomic moles of Li indicated by y to a basic metal salt represented by the general formula (IV)
Ni1xe2x88x92xCox1Nx3(OH)2(1xe2x88x92x+x1)+3xc3x973xe2x88x92nz(Anxe2x88x92)z.mH2Oxe2x80x83xe2x80x83(IV)
[wherein N represents at least one element selected from the group consisting of Al, Fe and Mn, in this case M in the general formula (I) contains both the N and B, and if the content of B therein is indicated by x4, x represents 0 less than x 0 less than xxe2x89xa60.5, x1 represents 0 less than x1 less than 0.5, x3 represents 0 less than x3xe2x89xa60.3xe2x88x92x4, x1+x3+x4=x, Anxe2x88x92 represents an anion having a valence of n (n=1xcx9c3), and z and m represent positive numbers respectively satisfying the ranges of 0.03 xe2x89xa6zxe2x89xa60.3, 0xe2x89xa6m less than 2] in an aqueous medium, to form a slurry, spray or freeze drying the formed slurry and heating the spray or freeze dried product at a temperature of about 600xc2x0 C.xcx9c900xc2x0 C. for 4 hours or more in an oxidative atmosphere.
As the water soluble lithium compound and the basic metal salt which may be represented by the general formulae (II), (III) or (IV) (hereinafter, referred to as xe2x80x9cthe basic metal saltxe2x80x9d collectively), there may be employed one each containing an anion which is evaporated off during the heating.
As examples of the lithium compound, there may be selected one or more from among LiOH, LiNO3, Li2CO3 and hydrates thereof.
As examples of the boron compound, boric acid and lithium tetraboric acid may be preferably employed.
As example of Anxe2x88x92 in the basic metal salt, there may be selected from among the anions NO3, Cl, Brxe2x88x92, CH3COOxe2x88x92, CO32xe2x88x92 and SO42xe2x88x92.
In these compounds, LiOH as the lithium compound, boric acid as the boron compound and a basic metal salt wherein an anion is nitrate ion are used from the viewpoint of yield, reactivity, effective utilization of the resources and oxidation accelerating effect. The combination of these 3 kinds of compounds is particularly preferred from the viewpoint of battery characteristics.
As the basic metal salt which may be employed in the present invention, it is preferable that the basic salt having a specific composition that the size of the primary particle is fine as below 0.1 xcexcm when measured by the Scherrer""s method.
Also, it is preferred that this fine particle has a BET specific surface area of 10 m2/g or higher, preferably 40 m2/g or higher, more preferably 100 m2/g or higher. As to the BET specific surface area, if it is measured after the basic metal salt in an aqueous solution has been dried, as the very fine primary particles aggregates during the dry process, then BET specific surface area of the aggregate is measured. If the aggregation power is strong, nitrogen gas cannot enter into it and the value of BET specific surface area becomes small. Accordingly the basic metal salt which is practically reacted with a lithium compound in aqueous solution shows a high BET specific surface area so that the surface is highly reactive. However, BET specific surface area was set to 10 m2/g or higher from the above actual circumstances. The basic metal salt having such specific composition has a layered structure, and the chemical composition and the crystal structure where M is at least one of Al, Fe and Mn are similar to those of hydroxide of Ni1xe2x88x92xCox1Mx2. The chemical composition and the crystal structure where M is B are similar to those of hydroxide of Ni1xe2x88x92xCox1. And the chemical composition and the crystal structure where M is the combination of B and at least one of Al, Fe and Mn are similar to those of hydroxide of Ni1xe2x88x92xCox1Nix3. Moreover, in all cases, the basic metal salt is microcrystalline whose surface is highly active. When it is reacted with a lithium compound such as LiOH, an extremely desirable precursor of LiyNi1xe2x88x92xCox1Mx2O2 is formed.
Highly purified LiyNi1xe2x88x92xCox1Mx2O2 having an extremely high crystallinity at which the present invention aims can be obtained only when the basic metal salt having such a specific composition is used. The hydroxides in the above are inferior in the reactivity with the lithium compound to the basic metal salt. On the other hand, when the amount of an anion in the basic metal salt is increased, the basic metal salt deviates from the layered structure, and the anion acts inhibitively on the formation of LiyNi1xe2x88x92xCox1Mx2O2 during heating, thereby the desired compound having a high purity and an extremely high crystallization degree cannot be obtained.
The basic metal salt to be used in the present invention can be prepared by adding an amount of about 0.7xcx9c0.95 equivalent, preferably about 0.8xcx9c0.95 equivalent of an alkali based on Ni1xe2x88x92xCox1Mx2 salt, Ni1xe2x88x92xCox1 salt or Ni1xe2x88x92xCox1Nx3 salt under the condition below about 80xc2x0 C. to effect the reaction. Examples of the alkali to be used in the reaction include alkali metal hydroxides such as sodium hydroxide, alkaline earth metal hydroxides such as calcium hydroxide, amines and the like. In this connection, it is preferable that this basic metal salt is matured at 20xcx9c70xc2x0 C. for 0.1xcx9c10 hours after its preparation. Subsequently, any byproducts are removed by washing with water and the lithium compound is added, and the boron compound is further added for the purpose of preparing a complex oxide containing B.
For drying the slurry obtained by such a reaction, spray or freeze drying method is desirable. The spray drying method where drying can be instantaneously accomplished and the spherical particles can be obtained is preferred from the viewpoint of the spherical granulation nature and the uniformity of the composition (in dry process requiring some drying time, lithium migrates into the surface of particles to give a non-uniform composition).
The heating is effected at a temperature of 600xc2x0 C.xcx9c800xc2x0 C., preferably 700xc2x0 C.xcx9c750xc2x0 C. for 4 hours or higher, preferably about 4xcx9c72 hours; more preferably about 4xcx9c20 hours under an oxidative atmosphere (under the flow of oxygen). If the heating time is 72 hours or more, not only do costs increase but also it causes evaporation of Li thereby the proportion of trivalent (Ni+Co) to the total (Ni+Co) becomes rather low and the purity becomes bad.
In the known technique of the drying process, heating of at least 20 hours was required for Ni which is hard to convert into trivalent from divalent. In light of that fact, the process of the present invention which may be carried out even with a shorter heating time than 20 hours is very economical and advantageous.
The second process is a press molding process which is advantageous for the purpose of making the primary particle large and further making the bulk density high.
The dry product obtained by the spray drying or freeze drying process above-mentioned is press molded and then heated, whereby not only the size of the primary particle may be optionally set within the range of 1xcx9c30 xcexcm, but also there can be obtained the complex oxide having high bulk density, degree of crystallization and purity.
The spherical particle that is the spray dried product is an excellent powder with respect to flowability, molding and filling properties, and it is a good material to be pressed into a shape according to the conventional manner.
Although the pressure for molding may be varied depending on the pressing machine to be applied and the amount to be fed and is not limited particularly, usually a pressure about 500xcx9c3,000 kg/cm3 is suitable.
Pressing machine to be applied is not limited particularly and it may be one capable of pressing. However, tablet compressing machine, briquette, roller compactor may be suitably employed.
The density of the press molded product may be about 1xcx9c4 g/cc, preferably about 2xcx9c3 g/cc.
The press molding is very useful in that the moving distance among molecules becomes short and crystal growth during the heating is accelerated. Accordingly, it is not always necessary that the material to be subjected to the press molding is spray dried spherical particle product. The freeze dried product may also be used.
This press molded product can be heated as it is. The heating is effected at a temperature of usually 600xc2x0 C.xcx9c900xc2x0 C., preferably 700xc2x0 C.xcx9c800xc2x0 C. for a period of 4 hours or higher, preferably 10xcx9c72 hours under an atmosphere of oxygen.
The longer the heating time, the larger the size of the primary particle. Therefore, the heating time is determined depending on the desired size of the primary particle.
For accomplishing the heating in a short time, heating 2 times, pre-heating and after-heating may be applied. The slurry obtained by the process described previously is spray- or freeze-dried and the spray- or freeze-dried product is first pre-heated at a temperature of about 600xc2x0 C.xcx9c900xc2x0 C. for 0.5 hour or more (preferably 0.5xcx9c4 hours) under an oxidative atmosphere, the obtained pre-heating product is pulverized if necessary and pressed into a shape, and then after-heated at a temperature of about 600xc2x0 C.xcx9c900xc2x0 C. for 1 hour or more (preferably 4xcx9c48 hours) under an oxidative atmosphere. The total time required for the heating may be shortened by employing this process.
The thus obtained complex oxide which may be represented by the general formula (I) retains high battery capacity of 160xcx9c180 mAh/g even after 100 charging and discharging cycles and has an improved high temperature cycle property (stability) as is apparent from the Examples described later, and hence it may be effectively utilized as a positive electrode active material for a secondary battery.